It is well recognized that state-of-the-art wafer steppers use monochromatic exposure light based on mercury arc lamps and excimer lasers. This limits the spectral range necessary for chromatic correction, simplifying lens design. The high degree of coherence associated with narrow band width leads to the well known thin film interference effects in photoresists. This in turn produces undesirable variability in exposure energy coupling within the resist on partially reflective substrates containing topographical features and/or transmissive thin films of variable thickness.
Early techniques for dealing with this problem relied on the use of anti-reflective coatings (ARC) applied beneath the resist coating to absorb exposure light transmitted through the resist film. This prevents phase-coherent waves reflected from the substrate from mixing with waves reflected from the resist surface, reducing or eliminating interference. The use of these anti-reflective layers, while effective in controlling both bulk reflectance variability as well as interference effects, complicates lithography processing by imposing an additional subtractive film removal step. After exposure and development, the anti-reflective layer is typically removed leading to additional erosion and potential increase in etch bias.
More recent techniques rely on the reduction of surface waves from the resist film surface rather than the substrate, to prevent phase-mixing and interference. The technique, termed "anti-reflective coating on resist" (ARCOR) is based on the well known method of applying a quarter-wave optical thickness of material having a refractive index which is close to the square-root of the underlying material. Thus, for a photoresist with a typical index of refraction of about 1.7, applying a quarter-wave thickness of a coating with an index of refraction of 1.3, completely eliminates reflection from the resist surface, thereby eliminating interference with light reflected from the substrate. The problem with this technique is the difficulty in providing aqueous processable coating materials with sufficiently low refractive index values. While fluorocarbon polymers with index values close to 1.3 are available, these must be processed with exotic and expensive fluorocarbon solvents, complicating the aqueous based lithographic process. Aqueous processibility of fluorocarbon-based processability can only be attained by employing materials with a higher refractive index of about 1.4. This is, at best, a mere compromise.
The applicant is aware of several references which are listed below. The pertinence of these references to the current invention will be considered and discussed in the Summary of Invention section.
U.S. Pat. No 4,971,893 describes an acrylic coating to provide a "matte" finish on image bearing surfaces.
U.S. Pat. No 4,987,048 describes an acrylic binder for coloring materials in the photographic process.
U.S. Pat. No 4,465,767 employs acrylic pigment binders for optical recording systems.
British Pat. No. G.B. 2,130,430 teaches the use of acrylic polymers as substrate support materials for optical video disc.